Small 4 K cryocoolers for the cooling of low temperature superconducting (LTS) electronic systems are necessary for broader commercial, military, or space applications of such devices. Typically these cryocoolers have been either Gifford-McMahon (GM) cryocoolers or GM-type pulse tube cryocoolers that operate at frequencies of about 1 Hz. The efficiency of these cryocoolers ranges from 0.5 to 1.0% of Carnot, whereas 80 K cryocoolers often achieve efficiencies of about 15% of Carnot. The low efficiency of 4 K cryocoolers causes these cryocoolers to have large, noisy compressors with high input powers. The low operating frequency of the GM and GM-type pulse tubes also leads to large temperature oscillations at the cold end at the operating frequency of the cryocooler. The amplitude of the temperature oscillation decreases inversely with the cryocooler operating frequency.
Higher operating frequencies allow the use of Stirling cryocoolers or Stirling-type pulse tube cryocoolers, which have much higher efficiencies in converting electrical power to PV power. These frequencies are typically in the range of 30 to 60 Hz. The linear Stirling-type compressors (pressure oscillators) often use flexure bearings that eliminate rubbing contact and operate almost silently. However, these higher frequencies generally lead to greater losses in a 4 K regenerator unless the operating parameters are near optimum conditions. Recent regenerator modeling efforts have shown that the phase angle between flow and pressure at the cold end has a strong effect on the 4 K regenerator second law efficiency. In order to achieve an optimum phase of about −30° (flow lagging pressure) at the cold end, a phase of about −60° at the pulse tube warm end is required. Inertance tubes are typically used for phase shifting, but with the small refrigeration powers of interest for electronics cooling, phase shifts of only a few degrees are possible at 30 Hz, even with the inertance tube and reservoir at a low temperature of 30 K. A double inlet configuration with a secondary orifice between the regenerator and pulse tube warm ends can only provide a practical phase shift of about 30° before the lost work in the secondary orifice greatly reduces the overall efficiency. The double inlet approach also introduces the possibility of DC flow, which can reduce the efficiency.
Larger phase shifts with small acoustic powers can be achieved by the use of a warm expander or warm displacer at the warm end of the pulse tube. For single stage pulse tube cryocoolers or for two-stage pulse tube cryocoolers operating at about 1 Hz (GM-type), the warm end of the pulse tube operates at ambient temperature. A 4 K pulse tube may need to have the warm end at 30 K or lower to keep the efficiency of the pulse tube component high, at least for a high frequency of about 30 Hz. It would then be necessary to develop an expander that can operate at about 30 K.
In view of the foregoing, it would be desirable to provide a pulse tube refrigeration system having a room temperature phase shifter or expander.